As integrated circuit semiconductor devices acquire higher and higher circuit densities, extremely clean manufacturing environmental conditions are required to ensure that the semiconductor devices are free of contamination of impurity materials that cause imperfection and deterioration of the devices if introduced during manufacture of the devices. Such impurity metals for example include Fe, Cu, Cr, Ni, etc.
In particular, in the process of treating a semiconductor wafer at an elevated temperature, above 900.degree. C. or more, say, if the wafer is exposed to an atmosphere that contains contaminating impurity metals, the wafer is likely to adsorb the metals on the surface thereof, which metals are eventually taken in by the wafer, becoming a source of imperfection of the wafer.
Accordingly, jigs for use in the thermal processing of semiconductor wafers must be made from clean or pure materials, so that the jigs will not emit impurity materials onto wafers while the wafers are exposed to the jigs at high temperatures.
Thus, in the thermal processing of a semiconductor wafer or substrate for growing a layer on the surface thereof for example, it is customary to use jigs made from high purity quartz (SiO.sub.2) or high purity silicon carbide (SiC) which contains only negligible amount of impurity metals.
In the processes of growing a layer on the semiconductor wafer and in the heat treatment of the wafers, a furnace tube is often utilized, which consists of a quartz tube and a heater wound on the exterior of the quartz tube. For example, silicon wafers are placed on a quartz jig and the entire jig accommodating the silicon wafers are inserted in the furnace tube to thereby form oxidized layers on the wafers and/or to cause required impurities to be diffused in the substrates at high temperatures. Such a jig for the heat treatment is called a wafer boat.
Conventionally, such a furnace tube and a wafer boat as mentioned above are made from quartz since quartz is relatively easy to fabricate and not too expensive. For this purpose, a chunk of quartz is normally grown from fused quartz, which may be obtained from quartz powder by fusing it in mainly two different ways as described below:
(1) Fusing quartz powder by oxyhydrogen flame in a Bernoulli furnace; and PA1 (2) Fusing quartz powder in an electric furnace for vacuum fusion. The former method that uses oxyhydrogen flame is referred to as "oxyhydrogen flame fusion method", and the latter as "electric fusion method". Synthetic quartz or synthesis quartz which is grown epitaxially is also known. This quartz however, exhibits a disadvantageously large thermal deformation if it is heated to a temperature as high as 1000.degree. C. in the furnace, so that it is not practical to use it for material of a furnace or a thermal processing jig. PA1 a body made from a first type of quartz having high heat resistance; PA1 portions, adjacent to said silicon wafers, made from a second type of quartz containing a relatively small amount of metal impurities, wherein PA1 said body is separated from said silicon wafers by a predetermined distance. PA1 In the thermal processing jig described above, the first type of quartz for the body may be electric fusion quartz, while the second type of quartz may be either oxyhydrogen flame fusion quart or synthetic quartz. PA1 fabricating a body of the jig from a first type of quartz having high heat resistance; PA1 fabricating portions of the jig, adjacent to the silicon wafers, from a second type of quartz containing a relatively small amount of metal impurities; PA1 bonding said portions and said body together by flame-melting the contacting areas of said body and said portions.
The oxyhydrogen flame fusion quartz and electric fusion quartz are both made from the same natural resources such as silicate rock and silicate sands. Unfortunately, oxyhydrogen flame fusion quartz is inferior in heat resistance than electric fusion quartz and is likely to be deformed by heat when used in a thermal processing jig. This is due to the fact that oxyhydrogen flame fusion quartz is made from quartz powder by fusing it with oxyhydrogen flame, so that it contains a fairly large amount of OH radicals (as much as 100 ppm). On the other hand, electric fusion quartz, developed to overcome this disadvantage of oxyhydrogen flame fusion quartz, has a superior heat resistance, since it contains much less OH radicals (as much as 30 ppm) than the oxyhydrogen fusion quartz because the quartz powder is melted in a vacuum furnace. The heat resistance of quartz is strongly connected with the amount of OH radicals contained therein. The greater the number of OH radicals in quartz, the less heat resistance it has, and is therefore more deformable.
Detailed information on the heat resistance of quartz has been reported in various documents such as, for example, a book entitled "KOGYO ZAIRYOU", on July 1974 issued by NIKKAN KOUGYO SHINBUN and an article entitled "NEW GLASS FORUM", NEW GLASS, No. 14, 1987.
Because these two kinds of quartz are obtained from natural ores such as silica stone and silica sand, they contain relatively large amount of impurity metals. They cannot be regarded as "pure quartz". It is therefore necessary to prevent the impurities from being emitted from the quartz used in a thermal processing when the quartz is exposed to a high temperature during manufacture of semiconductor devices.
Japanese Patent Laid-Open Publication (KOKAI) Nos. SHO 55-126542 and HEI 3-84922 have disclosed use of vitreous quartz which is epitaxially synthesized and deposited on the inside of a cylinder made from electric fusion quartz and having high heat resistance. This arrangement is designed to prevent contaminants emitted, by means of a layer of the synthetic quartz, from the electric fusion quartz from reaching the silicon wafer placed in the cylinder.
Japanese Patent Laid-Open Publication (KOKAI) No. SHO 63-236723 discloses a combination of an electric fusion quartz tube and an outer layer which contains aluminum and is deposited on the exterior of the quartz tube for stopping the infiltration of impurities from outside of the tube.
The present inventors have disclosed in Japanese Patent Laid-Open Publication (KOKAI) No. HEI 6-69145 a furnace tube 19 as shown in FIGS. 1 and 2. The furnace tube 19 is provided on the exterior thereof with a winding heater 21 as seen in FIG. 1, and inside thereof with a thermal processing jig 11 made from quartz for supporting silicon wafers 12. The quartz jig 11 accommodating the semiconductor wafers 12 is placed in the furnace tube 19, where the wafers 12 are subjected to a thermal process to form, for example, an oxidized layer, or to diffuse required impurities into the wafer.
As shown in FIG. 1 as well as in FIG. 2A showing a radial cross section of the furnace tube 19, the furnace tube 19 includes a first layer 18 of heat resistive or refractory electric fusion quartz. The inner surface of the first layer 18 is covered with a second layer 17 of oxyhydrogen flame fusion quartz. This furnace tube 19 is designed to provide a major portion of the tube made from less deformable electric fusion quartz on one hand, and provide on the other hand a lining of less contaminating oxyhydrogen flame fusion quartz on the inner surface of the tube facing the silicon wafer 12, so that metal impurities liberated from the electric fusion quartz will be stopped by the oxyhydrogen flame fusion quartz.
The above mentioned Japanese Laid-Open Publication discloses a similar furnace tube 19 further having a third layer 20 of synthetic quartz which is epitaxially grown between the first layer 18 and the second layer 17, as shown in FIG. 2B. The third layer 20 is intended to reinforce the function of the second layer 17.
The present inventors have proposed in the foregoing Laid-Open Publication an alternative thermal processing jig or wafer boat 11 made from quartz as shown in FIGS. 3 and 4. This thermal processing jig is also indicated by the same reference numeral 11 as the jig disposed in the furnace tube 19 shown in FIG. 1.
In this thermal processing jig 11, the entire surface of the heat resistive electric fusion quartz (first layer) 18 is covered with an oxyhydrogen flame fusion quartz (second layer ) 17, as shown in FIG. 3 and in FIGS. 4A and 4B in cross section.
A still further alternative has been discussed in the above Laid-Open Publication, in which a first layer 18 and a second layer 17 respectively, are intervened by a third synthetic quartz layer, as shown in FIGS. 4B and 4D. The third layer 20 is provided to enhance the function of the second layer 17.
The prior art furnace tube mentioned above has a rather simple configuration consisting of a generally hollow cylinder with one end thereof closed, so that it may be fabricated without any problem. It has been known that such configuration of the furnace tube 19 may effectively prevent deposition of contaminating metals from the furnace tube during heat treatments of the semiconductor wafers. However, a quartz jig such as a wafer boat 11 incorporates a multiplicity of constituent members bonded together by melting contacting portions thereof and hence has a rather complex configuration. Thus, it is very involved to fabricate the members in such a way that each of them has a first core layer, a second surface layer covering the first layer, and a third layer for covering the entire structure while maintaining an advantageous yet complex configuration of the boat, hence results in a very low manufacturing efficiency of the jigs.